1. Field of the Invention
This invention relates to liquid crystal composites and liquid crystal devices made from such composites.
2. Description of Related Art
Light valves having an electro-optically active element comprising a liquid crystal composite have been used in displays (both directly driven and active matrix), windows, and privacy panels. In a liquid crystal composite, plural volumes or droplets of a liquid crystal material are dispersed, encapsulated, embedded, or otherwise contained within a matrix material such as a polymer. Exemplary disclosures include Fergason, U.S. Pat. No. 4,435,047 (1984); West et al., U.S. Pat. No. 4,685,771 (1987); Pearlman, U.S. Pat. No. 4,992,201 (1991); Dainippon Ink, EP 0,313,053 (1989).
The liquid crystal composite is disposed between electrodes, which are respectively supported by substrates (e.g., glass or polymer). When no voltage (i.e., no electric field) is applied across the electrodes (the field-off state), incident light is substantially scattered and/or absorbed, creating a frosty or opaque visual appearance. When an appropriate voltage is applied across the electrodes (the field-on state), the liquid crystal composite changes its optical state to one in which incident light is substantially transmitted.
It is desirable that the composite have low light transmissivity in the field-off state (T.sub.off, as defined hereinbelow), to provide high contrast against the substantially transmissive field-on state. It is also desirable that the composite be switchable from the field-off to the field-on state by the application of as low an electric field as possible. This performance characteristic may be quantitated by the operating field E.sub.90 (also defined hereinbelow), which measures the electric field required to switch the composite into its substantially transmissive state.
A low E.sub.90 is desirable because, for a given applied voltage, a thicker layer of composite can be used (resulting in a lower T.sub.off). Also, in devices such as active matrix displays, the driver controlling the application or not of a voltage across the composite typically is a semiconductor device (e.g., a CMOS chip) having an operating voltage of only several volts. For the driver to successfully control the switching of the composite, the composite must have a low E.sub.90.
One method employed by researchers in the art to improve the electro-optical performance of a liquid crystal composite, whether that be T.sub.off, E.sub.90, or some other parameter, is to include an additive in the composite. Additives which have been used for one purpose or another include polyols, surfactants, polymers of various kinds, amphiphilic telomers, charge transfer complexes, plasticizers, polysilanes, alkyl alcohols, phenolic antioxidants, silicones, crown ethers, acrylates, methacryates, and silane coupling agents. Noteworthy disclosures in the genre include Wartenberg et al., U.S. Pat. No. 5,427,713 (1995); Wartenberg et al., U.S. Pat. No. 5,484,552 (1996); Wartenberg et al., U.S. Pat. No. 5,571,448 (1996); Raychem, WO 96/19547 (1996); Seiko-Epson, JP 3-288823 (1991); Seiko-Epson, JP 4-7518 (1992); Hitachi Cable, JP 7-140448 (1995); Hitachi Cable, JP 7-292361 (1995); Hitachi Cable, JP 7-294,890 (1995); Hitachi Cable, JP 7-294891 (1995); Hitachi Cable, JP 8-220,513 (1996); Hitachi Cable, JP 8-227070 (1996); Adomenas et al., Mol. Cryst. Liq. Cryst. 1992, Vol. 215, pp. 153-160; and Sumitomo Electrical Industries, EP 0,421,240 (1990).
However, even though some prior art additives may be successful at reducing E.sub.90 and/or T.sub.off, it is difficult to obtain an additive which consistently does so for many different combinations of liquid crystal compositions and matrix materials. Further, an additive which reduces E.sub.90 may have an adverse effect on T.sub.off.